Static list instead of synchronized cache for collection-types
Switched to using EnumMap for SetType and ListType. Added fallback to 'Collection'Type.getInstance(...) for custom DateType.
Added tests to make sure a custom type is returned and not null (as was the case).

Removed javadoc and added license header for CodecTest.
Swapped usage of Pairs to map of maps
Merge pull request #140 from olvesh/codec-prewarm-cache

Static list instead of synchronized cache for collection-types